During the servicing of oil and gas wells it is often necessary to pump fluids and slurries down the well at pressures that are greater than the ratings of the associated wellhead equipment such as valves, hangars, spools etc. The wellhead isolation tool or tree saver was developed for the purpose of pumping through wellhead valves and down the well tubing. Tree savers work on wells that are dead or under pressure and have tubing in them. However, there is a large number of wells that must be serviced which do not have tubing in them and the servicing must be done down the casing.
Wells of this type are also dead or have very little pressure and do not have any wellhead valves mounted on them. A tree saver will not work reliably on such wells because conventional tree savers make a seal in the tubing by forcing a nipple with a rubber on the end, into the tubing to thereby seal off the servicing pressures from the wellhead equipment. This is accomplished only because the tubing has a very small manufacturing tolerance with regard to the inside diameter in the area where the nipple seals. A well casing on the other hand has a wide manufacturing tolerance and it is difficult to get a nipple to seal in the casing for the pressures encountered.
Conventionally, the method of servicing down well casing has been to use what is known as a downhole packer. The purpose of the packer is to pump the fluid or slurry down the long string casing without the fluid having access to the tubing spool mounted on top of the casing head. To accomplish this, a packer is lowered into the long string casing on tubing using a service rig such as a Franks 100. The packer is then set by either tension or compression and by turning the tubing. The tubing at the top is then set up so that fluid can be pumped downwardly through the tubing, through the packer and into the lower part of the long string casing. The packer and service rig may be of any of several available makes.
Those skilled in this art will appreciate the fact that pumping fluid down one thousand feet of tubing takes much more power than to pump the fluid down the casing. Testing to make sure that the packer is set entails pumping fluid down the annulus and then taking the pressure up to see if the packer may leak. There is a significant percentage of packer failure in this type of service and pulling out and re-running the packer is very time consuming and very expensive. As an example, the cost to run a downhole packer of the type referred to above would be in the region of twenty-five thousand dollars. The tubing and packer must also be taken out after the job and this may entail a return trip to the well by the whole crew and rig if the servicing job was not done while the rig was on the site.